bpo-37929: IDLE: avoid Squeezer-related config dialog crashes (GH-15452)

bpo-37929: IDLE: avoid Squeezer-related config dialog crashes (GH-15452)

These were caused by keeping around a reference to the Squeezer
instance and calling it's load_font() upon config changes, which
sometimes happened even if the shell window no longer existed.

This change completely removes that mechanism, instead having the
editor window properly update its width attribute, which can then
be used by Squeezer.